The invention relates to scanning-based ionizing radiation detector arrangements for two-dimensional detection of an object.
Gaseous-based ionizing radiation detectors, in general, are very attractive since they are cheap to manufacture, can employ gas multiplication to strongly amplify the signal amplitudes, and provide for detection with high spatial resolution.
A particular kind of gaseous-based ionizing radiation detector is the one, in which electrons released by interactions between photons and gas atoms can be extracted in a direction essentially perpendicular to the incident radiation. Hereby, a strongly improved spatial resolution is achievable.
Such a detector comprises typically planar cathode and anode arrangements, respectively, and an ionizable gas arranged in the space formed between the cathode and anode arrangements. The detector is arranged such that a planar radiation beam from a radiation source can enter the detector sideways between, and essentially parallel with, the cathode and anode arrangements for ionizing the ionizable gas. Further, a voltage is applied between the electrodes for drifting, and optionally multiplying, electrons created during ionization of the ionizable gas. A readout arrangement is arranged in connection to the anode for detecting the charge induced by the drifted electrons.
The detector obviously provides for instantaneous one-dimensional imaging, but to perform two-dimensional imaging the detector, and optionally the radiation source, have to be moved in a direction traverse to the one-dimensional detector array relative to an object being examined while several readouts are recorded. Such scanning-based two-dimensional detection is however time consuming and is impractical if large areas should be imaged. Further, if the object being examined is a human or an animal there is a risk that the human or animal moves during scanning, which could make the image useless or at least severely reduce the image quality obtained.
To reduce scanning time a stacked detector arrangement has been proposed in U.S. Pat. No. 6,118,125 by Francke at al., with which multi-line scans can be achieved. The arrangement includes an X-ray source, which together with a number of collimator windows produce a stack of planar fan-shaped X-ray beams for irradiation of the object to be imaged. The beams transmitted through the object enter the stacked detectors, optionally through a number of second collimator windows, which are aligned with the X-ray beams. The arrangement is moved as a unit to scan an object, which is to be examined.
In some applications such as e.g. medical applications the area to be imaged may be as large as 50 cmxc3x9750 cm, and the present inventors have noticed that a stacked detector arrangement as the one described in U.S. Pat. No. 6,118,125 for large area applications is very impractical to manufacture and use. Manufacturing tolerances are difficult to hold and to manufacture high-resolution detector units in volumes calls for a high level of efficiency, uniformity and quality.
A main object of the invention is therefore to provide a scanning-based ionizing radiation detector arrangement for two-dimensional detection of a large object with high spatial resolution.
In this respect there is a particular object to provide such a detector arrangement, which is suitable for volume production and still can produce large high-quality images, e.g. for medical examinations.
A further object of the invention is to provide such a detector arrangement, which comprises a plurality of line detector units in a dense matrix to shorten scanning time and distance.
A yet further object of the invention is to provide such a detector arrangement, which is reliable, accurate, inexpensive, and which has a long lifetime.
A still further object of the invention is to provide such a detector arrangement, which is capable of mitigating the problems caused from unusable dead channels (i.e. individual readout elements of the readout arrangement) by means of using more than one line detector unit to scan the same area of the object, also referred to as oversampling.
A yet further object of the invention is to provide such a detector arrangement, wherein movement blurredness can be minimized by means of recording short snapshots of each portion of the object by individual line detector units, where a possible movement of the object during a limited period of time, e.g. a heartbeat by a patient under investigation, only will affect a limited number of line images and not the complete two-dimensional image as is obtained by prior art two-dimensional detectors.
A still further object of the invention is to provide such a detector arrangement, wherein the effect of any movement blurredness can be further reduced by means of oversampling, i.e. recording a plurality of images at each location such that each portion of the two-dimensional image of the object is built up by contributions from several line images recorded at different times, where the object is most probably not moving during all of the several line image recordings.
A yet further object of the invention is to provide such a detector arrangement, wherein a plurality of line detector units are arranged in a matrix to provide for an overlap between channels (i.e. readout elements of the line detectors) located at the far edges to reduce the effect of possible edge phenomena, e.g. lower sensitivity at the far edges of the line detectors.
These objects, among others, are attained by detector arrangements as claimed in the appended claims.
The inventors have found that by arranging smaller ionizing radiation detector units, well suited to be volume produced with high precision, in a two-dimensional array, a scanning-based detector arrangement for highly resolved two-dimensional imaging of large objects, such as breasts in mammography examinations, is provided.
Further characteristics of the invention, and advantages thereof, will be evident from the detailed description of preferred embodiments of the present invention given hereinafter and the accompanying FIGS. 1-8, which are given by way of illustration only, and thus are not limitative of the present invention.